1. Field of the Invention
The present invention relates to a method for producing a colored anodic oxide film on an aluminum or aluminum base alloy article (hereinafter referred to, for brevity, as "aluminum"). More particularly, it relates to an improved method for producing a green-colored anodic oxide film on aluminum comprising an alternating current anodization of aluminum, an immersion of the anodized aluminum in an aqueous solution containing copper ions and an acid, and an aftertreatment.
2. Description of the Prior Art
Known methods for producing a colored anodic oxide film on the surface of aluminum include the following:
1. ANODIC OXIDATION OF ALUMINUM IN AN AQUEOUS SOLUTION CONTAINING AN ORGANIC ACID (FOR EXAMPLE, AS DISCLOSED IN U.S. Pat. No. 3,031,387);
2. ALTERNATING OR DIRECT CURRENT ELECTROLYSIS OF PREVIOUSLY ANODIZED ALUMINUM IN AN AQUEOUS SOLUTION CONTAINING METALLIC IONS (FOR EXAMPLE, AS DISCLOSED IN U.S. Pat. Nos. 3,382,160 and 3,761,362); and
3. DYEING AN ANODIC OXIDE FILM ON ALUMINUM WITH A DYE OR PIGMENT.
In methods (1) and (2), the colors obtained are limited and a green-colored oxide film cannot be obtained. Method (3) provides a wide range of colors; however, the product has poor resistance to weathering and cannot be used as building materials for outdoor uses.
On the other hand, there are known methods for producing a green-colored anodic oxide film on aluminum which has good resistance to weathering. One method comprises anodically oxidizing aluminum in an aqueous solution containing sulfuric acid and copper sulfate with an alternating current superimposed on a direct current (Japanese Patent Publication No. 27,490/1974). In this method, however, deposition of copper on a cathode easily occurs due to the use of the alternating current superimposed on the direct current, which not only leads to ageing of the electrolytic bath within a relatively short time but also to difficulty of bath control. This method is, therefore, sometimes disadvantageous on industrial scale operation from the economical point of view.
Another method for producing a green-colored anodic oxide film comprises subjecting aluminum to an alternating current anodization, with the aluminum as one electrode, in an aqueous sulfuric acid solution containing a water soluble metal compound (U.S. Pat. No. 3,717,555). This method provides a green-colored anodic oxide film having good resistance to weathering when a copper salt is used as the water soluble metal compound and a sealing treatment is performed after the alternating current anodization. This method, however, includes the following various problems which must be taken into account. The operational efficiency of the method is poor because only one electrode is aluminum and growth and coloration of the anode oxide film proceed at the same time due to the electrolysis; while the simultaneous progress of these two processes is very advantageous, the depth of the resulting coloration is limited by the film thickness. Further, the film formation efficiency is lower in alternating current anodization than in a conventional direct current anodization. Therefore, while it is possible to deepen the green shade by increasing the film thickness, when the desired thickness is increased beyond a certain level the operational costs become high, and, further, the surface of the resulting anodically oxidized film on the aluminum becomes very rough, which cannot provide a sound film. Finally, this method produces a clear green shade only with relative difficulty and it sometimes produces a yellowish, pale green shade unless operational conditions are strictly controlled.
A further method is disclosed in Metal Finishing Journal, April 1974, pages 80 - 84, wherein a green-colored anodic oxide film is obtained by subjecting aluminum to an alternating current anodization in an aqueous sulfuric acid solution and then immersing the aluminum in an aqueous copper sulfate solution followed by a sealing treatment. This method also provides an anodic oxide film of excellent resistance to weathering, but the green shade obtained is relatively pale, and, in most cases, strongly yellowish, because the immersion bath contains only copper sulfate. Further, shade reproducibility is very poor.
On the other hand, it is known that a sound, thick, green-colored oxide film can be obtained by subjecting aluminum to direct current anodization and then to an alternating current anodization and immersing the anodized aluminum in an aqueous solution containing a copper salt followed by a sealing treatment (Japanese Patent Publication No. 14,624/1975). This method, however, yields a deep shade only with difficulty.
As described above, there are well known methods which produce green-colored anodic oxide film having excellent resistance to weathering, but they all have problems in practical use.